If, in the context of the present invention, the term “optical surfaces” is used, this is to be understood to mean all such surfaces of optical components, as, for example, surfaces, in particular aspheric surfaces or free-form surfaces, of spectacle lenses, mirrors, plastic material optics, etc.
An apparatus and a method of the type specified at the outset are known from document DE 102 48 105 A1.
Spectacle lenses are conventionally manufactured from a blank by chip-removing machining of the so-called prescription surface or surfaces. The optically relevant shape of the spectacle lens is thus determined. Finally, the spectacle lens is polished, however, the polishing shall not effect a noticeable change of the optical characteristics.
For polishing a surface of a spectacle lens, a polishing head is conventionally used having a polishing tool, the polishing surface of which being at least approximately adapted to the shape of the surface of the spectacle lens to be polished. The polishing tool and/or the spectacle lens are gimballed, in particular by means of a ball joint, and are guided relative to one another along a predetermined motional sequence, mostly with the assistance of multi-axis robots.
Due to the relatively simple shape of the surface to be polished, it presents much less of a problem for polishing spheric or toric spectacle lenses to find an appropriate polishing tool of complementary shape that may be guided over the surface with relatively simple motional sequences, and without effecting unwanted deformations. Due to the high number of potential spheric or toric spectacle lenses it is only necessary to have a corresponding plurality of polishing tools at hand.
In this context, various groups of polishing tools have become known.
In a first group of such polishing tools (DE 101 00 860 A1; EP 0 567 894 B1), a rigid polishing member is always used, which is once for ever adapted to the shape of the surface to be polished, and, hence, may be used only for that particular surface.
In a second group of such polishing tools (DE 44 42 181; DE 102 42 422), a polishing member is used which, in operation, is rigid, however, which is initially transformed into a plastic state, for example by warming, so that it may adapt to any conceivable surface in that plastic state, before it again solidifies.
These two groups of polishing tools, hence, have in common that they are rigid in operation and, therefore, may be used only for polishing regularly shaped surfaces.
In a third group of polishing tools (EP 0 804 999 B1; EP 0 884 135 B1; DE 101 06 007 A1), a polishing body is provided which may be deformed also during operation. The deformability is affected by a bundle of parallel metallic rods which, at one end thereof, are journaled on an elastic membrane, and which may be displaced individually. The integral surface defined by their terminal surfaces at their other end is adapted to the shape of the surface to be polished.
These polishing tools, on the one hand, have the disadvantage that the membrane, as any such membrane, has a function of elasticity in which the center is the softest point with the elasticity decreasing in a radial outward direction, i.e. the membrane becomes stiffer close to its rim, or, the elasticity function has an increasing gradient. This, however, is disadvantageous for polishing tools of the type of interest in the present context, as was found out in the scope of the present invention, because this elasticity function gives rise to substantial deviations in shape. On the other hand, these polishing tools have the disadvantage that the displacement of the rods gives rise to mechanical friction, such that dynamic polishing processes may not be executed in practice.
In a fourth group of polishing tools (EP 0 779 128 B1; Patent Abstracts of Japan re. JP 08-206 952 A), polishing members are used having a pneumatically deformable polishing body. In that case, however, one has the same disadvantages in connection with an unfavorable elasticity function.
In a fifth group of polishing tools (DE 101 06 659 A1; DE 102 48 105 A1; DE 102 48 104 A1; US 2003/0017783 A1; WO 03/059572 A1), a member from an elastic material is provided in a polishing tool between a rigid base member and the polishing lining.
In these prior art polishing tools, however, the axial thickness of the elastic member is constant and the material of the elastic member is homogeneous. Accordingly, the elasticity is constant in a radial direction.
Insofar, with regard to prior art polishing tools for the machining of optical surfaces, in particular of spectacle lenses, one may state that the radial function of the pressure stiffness either increases in a radial outward direction, or is constant.
For relatively simply shaped surfaces (spheric or toric surfaces), this is sufficient. However, for the polishing of aspheric or non-point-symmetric free-form surfaces, respectively, such polishing tools may not be used without incurring problems.
Such free-form surfaces are conventionally also polished by means of numerically controlled polishing machines or polishing robots. In these machines, the polishing tool is guided over the spectacle lens surface to be polished by means of CNC. The polishing head drives the polishing tool mostly in a rotational movement, and, concurrently, applies same under pressure against the surface to be polished.
Aspheric or non-point-symmetric surfaces have curvatures which change over the surface. The polishing tool, during the polishing machining, moves over at least a portion of this irregularly curved surface. Therefore, it must be able to adapt with its elasticity to the prevailing local curvature, namely such that the polishing pressure is constant, if possible, over the contact surface. Only then one has a predeterminable constant removal of material, and the polished surface becomes entirely even. If this cannot be guaranteed, and the polishing pressure varies over the contact surface, then the desired aspheric surface topography is deformed and, consequently, its optical quality is reduced. Such deformations occur with prior art polishing tools in conventional production processes and, therefore, must be compensated stepwise, i.e. with iterative post-processing methods. This, however, is time and cost consuming.
With regard to the general prior art of polishing tools, one should mention DE 296 08 954 U1. This document describes an adaptive polishing head for being chucked in rotating tools. The polishing head comprises a base member being coated with a polishing material. The base member may consist of a soft, extremely elastic material, for example foam rubber. The polishing head, in an axial sectional view, has the shape of a mushroom, a cone or a ball, which means that it is thinner in the peripheral area, as compared to its center. Therefore, it is harder in its peripheral area.
A similar polishing head is also disclosed in U.S. Pat. No. 3,043,065. this prior art polishing head is mushroom-shaped and, hence, is likewise harder in its peripheral area, as compared to its center.
Finally, Patent Abstract of Japan re. JP 61-103 768 A also describes a polishing head of likewise mushroom-shaped figuration. This polishing head is subdivided into three concentric areas consisting of the same material, however, having air bubbles embedded therein in different concentrations. The central area has the maximum density of air bubbles, such that the effectively removed surface is at a minimum. It is at a maximum in the peripheral area.